1. Field of the Invention
The present invention is in the field of optical interferometers and components, and, in particular, is in the field of broadband light sources for interferometers, rotation sensors, and the like.
2. Description of the Related Art
A Sagnac interferometer comprises an optical loop, typically of optical fiber, that is used to sense rotation of an object onto which the loop is mounted. Briefly, such interferometers operate by dividing the optical energy from a light source into two substantially equal beams of light and causing the two beams of light to propagate around the loop in opposite directions. The two beams of light are combined after passing through the loop and the changes in intensity of the combined light caused by interference of the two beams is detected. In accordance with the well-known Sagnac effect, rotation of the object and thus of the loop of fiber causes changes in the relative phase between light propagating in the two directions which in turn causes the detected intensity to change. The rotation rate of the loop can be determined from the detected changes in the intensity. See, for example, U.S. Pat. Nos. 4,410,275; 4,529,312; 4,637,722; 4,671,658; 4,687,330 and 4,836,676.
With respect to Sagnac interferometers, it has been found to be advantageous to use a broadband light source to provide the light introduced into the loop of optical fiber. See, for example, U.S. Pat. No. 4,637,025 wherein a super radiant light source is described. The light source in U.S. Pat. No. 4,637,025 operates by introducing a pump signal into a single-mode optical fiber having a core doped with an active fluorescent material such as neodymium or other rare earths. The pump light has a sufficient intensity to cause amplification of spontaneous emission of photons by the fluorescent material. In one embodiment (FIG. 1), pump light is input into the optical fiber via a lens. In the second of the two embodiments, the pump light is introduced via a dichroic lens that is transparent to the pump light and highly reflective of emitted light. The pump light is absorbed by the fluorescent material and excites the electrons therein to higher energy states resulting in the emission light when the electrons transition to lower states. Because of the random manner in which the spontaneous emissions occur, the amplified emitted light is effectively spontaneous fluorescence and temporally incoherent.
The two embodiments in U.S. Pat. No. 4,637,025 generate light that is emitted in all directions in the core of the fiber. A portion of the light generated in the core propagates directly out the output end of the fiber. A second portion of the light propagates toward the input end of the fiber. In the first embodiment, the light reaches the input end of the fiber. Although the input end does not have a reflector, as in the second embodiment, surfaces in the optical path cause a portion of the generated light to be reflected back into the fiber. In the second embodiment, the dichroic reflector is included to specifically reflect the generated light that propagates toward the input end portion back toward the output end portion.
No reflector is provided at the output end of the fiber in U.S. Pat. No. 4,637,025 so that laser oscillations are purportedly prevented. See, for example, Column 5, lines 1-5, of the patent. Although there is no intent to support laser oscillations in such a light source, it has been found that when such light sources are used in combination with Sagnac interferometers, for example, the optical fiber loop of the interferometer acts like a mirror. That is, the light entering the fiber loop propagates around the loop and exits propagating in the opposite direction to the entry direction (i.e., back towards the light source). A portion of the exiting light re-enters the fluorescent optical fiber. The dichroic reflector (FIG. 2) at the input end of the optical fiber (FIG. 1) reflects this return light. Thus, it can be seen that an unintentional laser oscillation can occur because of the interaction of the reflection at the input end of the fluorescent fiber and the "reflection" caused by the optical fiber loop. The laser oscillations are unacceptable for many applications.